The present invention relates to a structure of a nuclear reactor core and, more particularly, to a structure of a reactor core which can facilitate the operation of the nuclear reactor.
In the existing boiling water reactor (BWR), the reactivity and the power distribution in the core are controlled by means of control rods, flow rate of the coolant through the core and the gadolinea (Gd.sub.2 O.sub.3) which is a burnable poison.
In a construction of a reactor core which provides reactivity and power distribution control relying upon the control rods, an extraordinarily large change in power peaking is caused when the control rods are extracted, resulting in a deteriorated soundness of the fuel rods. More specifically, the fuel assembly is constituted by a plurality of fuel rods each of which consists of UO.sub.2 pellets (fuel pellets) packed in a cladding tube. As the power of the reactor is increased, the volume of the UO.sub.2 pellet is increased due to swelling and thermal expansion so that the diameter of the UO.sub.2 pellet is increased at both ends thereof. Consequently, the UO.sub.2 pellet is deformed so as to contact at its central portion. When the deformed UO.sub.2 pellet contacts the wall of the cladding tube, a Pellet Cladding Mechanical Interaction (PCMI) is caused, often incurring a danger of breakage of the cladding tube. The possibility of the breakage of the cladding tube is large especially when the withdrawal of the control rods in the core is made while the reactor is operated to produce a high power.
A countermeasure which has been taken conventionally for securing the soundness of the fuel rod will be described hereinunder with specific reference to a boiling water reactor. The core of the boiling water reactor is usually constituted by fuel assemblies having an uniform axial enrichment distribution. The increase of the power of this reactor is initiated by withdrawing the control rods, which have been inserted into the core, until the density of the control rods in the core reaches 20%. This density of the control rod corresponds to the linear heat generation rate at which the PCMI is commenced, and the withdrawal of the control rods beyond this control rod density may incur the breakage of the fuel rods. It is therefore essential to stop the withdrawal of the control rods when the density thereof in the core has decreased to 20%. Then, the flow rate of the cooling water flowing through the reactor core is gradually increased to increase the power to a preselected level at a rate smaller than the critical rate as shown in U.S. Pat. No. 4,057,466 at which the PCMI is caused. The reduction of the power of the nuclear reactor due to an elapse of time is prevented, as disclosed in U.S. Pat. No. 762,248, by an increase of the flow rate of the cooling water so that the power of the nuclear reactor is maintained at a preselected level. When the reactor core is operated with a constant density of control rods, an uneconomical exposure assymmetry is caused. In order to avoid this, it is necessary to effect a change of the control rod pattern as shown in U.S. Pat. No. 762,248 once in a period of from a month to two months. The change of the control rod pattern cannot be made when the reactor is operated at high power, because such a change necessitates the withdrawal of the control rods. Therefore, the change of the control rod pattern is made by withdrawing the control rods while reducing the flow rate of the cooling water and decreasing the power down to the level below the linear heat generation rate at which the PCMI is commenced. Then, after the change of the control rod pattern, the power of the nuclear reactor is gradually increased to the preset level. During this step, in order to minimize the rate of change in the thermal load on the fuel rods during the increase of the power, the power is increased at an extemely small rate by increasing the flow rate of the cooling water. The above described manner of operation is extremely complicated and troublesome.
The specification of U.S. Pat. No. 3,799,839 and Japanese Patent Publication No. 12793/1976 disclose an example of a reactor core structure in which the concentration of gadolinea is varied in the axial and radial directions of the core. Although this arrangement considerably flattens the axial and radial power distribution in the core, the operation of this core still requires the aforementioned change of the control rod pattern, causing various inconveniences as stated before in connection with the power control by control rods.